A voltage regulator generates a regulated output voltage VOUT from an input voltage VIN. Ideally, the voltage regulator maintains the output voltage VOUT within regulation despite varying load current. The most desirable attributes of the voltage regulator include immediate response, inherent power supply stability, and fixed frequency. There are several methods of controlling the voltage regulator. PWM (pulse width modulation) control, hysteretic control and COT (constant on time) control are the most common control methods used in the voltage regulators, and each of them has its advantages and disadvantages.
The voltage regulators with PWM control work with fixed frequency, wherein the input is connected to the output according to the pulse width of the switching signal. By adjusting the pulse width of the switching signal, the output voltage is regulated. The drawback of the voltage regulator with PWM control is that it should be slowed down to prevent circuit instabilities and could not achieve fast transient response.
The voltage regulators with hysteretic control feedback the output voltage to a window comparator. With an upper threshold which is corresponding to the maximum value of the output voltage and a lower threshold which is corresponding to the minimum value of the output voltage, the feedback signal is maintained within the upper and the lower bounds of the window, and the output voltage is then maintained in a preset range. The voltage regulator with hysteretic control has fast transient response and inherent stability, but its frequency is varying.
The voltage regulators with constant on time control have the advantages of hysteretic control and seamless transition when the load changes, but the frequency of COT control is still variable with the input voltage and the output voltage because the off time is variable.
FIG. 1 schematically shows a prior art voltage regulator 100 with hysteretic control. The voltage regulator 100 comprises a hysteretic control circuit 103 that may be a window comparator. An output voltage feedback signal 112 is coupled to the window comparator 103. The window comparator 103 compares the output voltage feedback signal with an upper threshold and a lower threshold of the window comparator 103, and generates signals to toggle a main switch to supply power to the output based on the comparison.
FIG. 2 shows an example timing diagram of the output voltage waveforms in a voltage regulator under different conditions. In FIG. 2, all waveforms represent ripples of the output voltages for different input voltages (VIN) and a set output voltage VOUT for a buck regulator, which is a typical application for the voltage regulator. In FIG. 2, assuming waveform “a” has a higher value of VIN than the other waveforms (“b”, “c”, and “d”). Persons of ordinary skill in the art should know that the rising slopes of the ripples are proportional to (VIN-VOUT)/L, and the falling slopes of the ripples are proportional to −VOUT/L, wherein L represents the inductance of the output inductor. In addition, Vrip represents the ripple value of waveforms “a” and “b.” As is seen from FIG. 2, the rising slope of the waveform “a” is steeper than the rising slope of the waveform “b”, which means the waveform “a” has a higher input voltage; and the falling slopes of the waveforms “a” and “b” are same because of their same output voltage. Thus the waveform “a” needs less time to traverse upward through the window than the waveform “b” (Ts1<Ts2), and the frequency of the waveform “a” is higher than the frequency of the waveform “b.” It is concluded that the frequency of the voltage regulator 100 with hysteretic control is varied with the input voltage when the output voltage is set. Waveforms “c” and “d” are supposed to have same cycle time with waveform “b” because of their same input voltages and output voltages. But in FIG. 2, waveform “c” has a lower maximum value Vmax′ of the output voltage than the lower maximum value Vmax of waveform “b”, so waveform “c” takes less time to traverse upward through the window than the waveform “b.” Waveform “d” has a higher minimum value Vmin′ than the minimum value Vmin of waveform “b”, so waveform “d” takes less time to traverse upward through the window than the waveform “b”, too. The cycle time of the waveforms “c” and “d” may be the same with the cycle time of the waveform “a” which has a higher input voltage, if a suitable maximum value of the waveform “c” and a suitable minimum value of the waveform “d” are selected. So by varying either the maximum value or the minimum value of the output voltage or both of them, the fixed frequency can be achieved even when the input voltage is varying.
The present disclosure provides a voltage regulator with adaptive hysteretic control.